An electret or electret structure is a structure comprising a dielectric material and a quasi-permanent electrical charge, so that the electret structure exhibits either a quasi-permanent electrical charge or dipole polarization. An electret may contain positive or negative charges in charge traps on the dielectric's surface (a surface charge) or in the dielectric's volume (a space charge) or it may contain oriented (aligned) dipoles. An electret can, for example, be formed by polarization in the presence of a high electric field, by cooling a suitable dielectric material within a strong electric field after having it heated above its melting temperature, or by the application of corona or electron beam injection. The theory and applications of electrets have been studied in the past decades.
When an electret is exposed to e.g. a pressure wave, a thermal wave, a mechanical distortion or a light wave, a signal can be produced in an external circuit. Therefore, electrets can be used to sense pressure, temperature, light or sound. Electrets are used in a number of applications such as air filters, radiation dosimeters, transducers such as relays and microphones, and sensors.
Both organic and inorganic materials may be used for forming electrets. Silicon dioxide and silicon nitride based materials are interesting inorganic electret materials for MEMS applications because of their compatibility with MEMS technology.
A patterned electret structure is an electret structure with a charge pattern, i.e. an alternation or succession of regions with charges and regions without charges. In prior art, patterned electret structures are formed by patterning one or more dielectric layers and (fully) charging these one or more dielectric layers. In micromachined devices, there is often a need for regions with charges that are alternating with regions without charges. The regions with charges are created by providing an electret. In between the regions with charges, only substrate is present.
FIG. 1 shows a prior art electret structure 11 on a substrate 10 (e.g. semiconductor substrate such as a silicon substrate 10). The electret structure 11 shown in FIG. 1 comprises a first patterned dielectric layer 12 (e.g. a patterned silicon oxide layer), e.g. a dielectric layer 12 patterned so as to define a region where electret charges should be present, and a second patterned dielectric layer 13 (e.g. a patterned silicon nitride layer), e.g. patterned at the same location as the first patterned dielectric layer 12. At the interface between the first patterned dielectric layer 12 and the second patterned dielectric layer 13, charges, such as for example positive charges 14, are accumulated and trapped. The positive charges 14 may be injected for example by corona charging or by other methods known by a person skilled in the art. In addition, as illustrated in FIG. 1(a), distributed trapped charges 15 (space charges) may be present in the first patterned dielectric layer 12 and/or in the second patterned dielectric layer 13. These distributed trapped charges 15 in the dielectric material are less stable than the charges 14 at the interface of the dielectric layers, and may be removed by means of an appropriate treatment, such as, for example, an annealing step (at a temperature above the main Thermally Stimulated Discharge peak for space charges, for example at a temperature of 450° C.). This results in a structure as illustrated in FIG. 1(b), where the patterned electret structure 11 comprises two patterned dielectric layers 12, 13 defining the region where charges are trapped, with substantially only at the interface trapped charges 14. In alternative patterned electret structures as known in the art, a single patterned dielectric layer, such as a patterned silicon dioxide layer or a patterned silicon nitride layer, may be used instead of the double layer structure shown in FIG. 1, as for example illustrated in FIG. 2.
An important aspect of electrets or electret structures is their long-term charge stability.
Treatment at elevated temperatures has been used for charge stabilization of electrets, as, for example, reported by Leonov V. et al in “Stabilization of positive charge in SiO2/Si3N4 electrets,” IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 13, 2006, pp 1049-1056. Surface treatment of electrets with a hydrophobic material such as octadecyl dimethyl (dimethilamino) silane or hexamethyldisilazane (HMDS) helps to stabilize the charges and thus to improve charge retention in the entire electret layer or at a predetermined part of the electret layer.
The charge stability of prior art patterned electret structures deteriorates with decreasing dimensions of the patterns or structures, more particularly with decreasing width W of the electret structure 11. This is, for example, described for silicon dioxide electret structures by T. Genda et al in “High power electrostatic motor with micropatterned electret on shrouded turbine,” Proceedings of the 13-th International Conference on Solid-State Sensors, Actuators and Microsystems, Jun. 5-9, 2005, pp. 709-712. The charge stability of the micropatterned electrets substantially deteriorated when the width decreased below several tens of microns. A leakage current 16 through the electret surface is considered as the dominant reason for the surface potential decay, as schematically illustrated in FIG. 2 (for positive charges, as an example). This effect is more pronounced for narrower structures. The charge stability can be improved by means of a surface treatment of the electret surface, leading to terminating the electrets by fluorinated silane coupling agents, for example, but this improvement is not sufficient for practical applications with small patterns (such as 10 micrometer wide patterns or smaller, for example), e.g. in MEMS devices.
Furthermore, it was found that, when charging a prior art patterned dielectric structure by means of corona charging, e.g. for forming a patterned electret structure, part of the charges are deviated near the edges of the patterned dielectric structure, as illustrated in FIG. 3. As a result, the smaller the width of the electret structure, the less potential can be created on the electret structure. This is related to the presence, during electret charging, of an electrically conducting or semiconducting substrate that is either grounded or has a lower potential than the electret structure. As illustrated in FIG. 3, corona charging then leads to a charge distribution with a smaller charge density near the edges of the electret structure as compared to the charge density in the center portion of the electret structure. This effect is more pronounced for electret structures with smaller dimensions. It was found that for small electret patterns (e.g. for patterns with a width lower than 400 micrometer) corona charging is very difficult or even impossible. This is illustrated in FIG. 4.
FIG. 4 shows the potential that was measured for prior art electret structures with different widths, wherein the electret structures were charged by means of corona charging. The electret structures have a rectangular shape and comprise a 550 nm thick silicon oxide layer on a silicon wafer. FIG. 4 shows measurements of a prior art patterned electret structure comprising parallel ‘lines’ of electrets with decreasing width and distance. This is schematically illustrated by the black/white pattern at the bottom of the figure (the black regions corresponding to a charged region). The horizontal axis shows the distance from an edge of this structure. In the direction indicated with “distance” in FIG. 4(a) and FIG. 4(b), the width of the electret structures decreases. FIG. 4(a) shows the measured potential immediately after charging. FIG. 4(b) also shows the measured potential two days after charging. From the measurement results shown in FIG. 4(a) it can be concluded that at a width below 400 micrometer no charge is created. Furthermore, as can be concluded from the measurement results of FIG. 4(b), discharging of the electret structures is dependent on the line width of the structures. The smaller the line width of the electret structures, the faster they lose charges.
For high-resolution patterning of electrets, microelectronic technologies are used. For example, a photoresist mask is used and the electret layer is etched to remove the electret in regions that are not protected with a photoresist. However, these microelectronic processes do not provide stable electrets: the charge stability depends on the feature size, and small or narrow patterned structures are discharged very fast. Therefore, prior art methods of electret patterning may not provide sufficient charge stability in narrow electret structures (such as 10 micrometer to 100 micrometer wide lines, for example) for use in commercial devices and systems.